The respiration of an individual or patient may be monitored for a variety of reasons. For example, knowledge about a patient's respiration may assist a physician in assessing the patient's stability during surgery and recovery after surgery. Another rapidly growing field in which information about an individual's respiration may be of value is the field of sleep therapy.
Sleep therapy frequently involves, in part, the monitoring and analysis of an individual's respiration to improve the individual's sleep patterns. For example, when some individuals are asleep, their respiration becomes so erratic that they are frequently awakened from their sleep. In some instances, these awakenings may occur so often that the individual is left exhausted in the morning. Given the brief nature of the awakenings, however, they may not even be remembered. As a result, the nature of the sleep disorder may go undiagnosed. By monitoring the individual's respiration during the course of the night, however, the nature of the problem may be readily identified and treated.
A variety of techniques have been developed for use in monitoring respiration. Of particular interest, however, is an approach that senses the temperature of the air inhaled and exhaled by the individual. As will be appreciated, in most environments, the air breathed in by the individual will be at a lower temperature than the air breathed out. Thus, by monitoring temperature changes occurring in the individual's respiration path, an indication of respiration can be obtained.
Several different types of sensors are often used in thermal respiration monitoring systems. For example, either a thermistor or thermocouple can be employed as the temperature-sensitive element. Addressing first the thermistor-based system, a thermistor is a device whose resistance changes with temperature. A thermistor-based respiration monitoring system typically includes a thermistor, positioned in the respiration path, and a monitor that applies, for example, a small voltage to the thermistor. As the individual breathes, the temperature of the thermistor changes, causing the thermistor's resistance to change and altering the current flowing through the thermistor. The monitor then senses the current change and translates it into an indication of the change in respiration.
Unlike a thermistor, a thermocouple actually generates a thermoelectric potential that is proportional to the temperature difference between a pair of thermocouple junctions. A thermocouple-based respiration monitoring system employs a "hot" thermocouple junction in the individual's respiration path and a "cold" thermocouple junction outside the respiration path. These junctions produce a thermoelectric potential that is proportional to the temperature difference between the junctions. The thermocouple-based respiration monitoring system includes a monitor that translates changes in the thermoelectric potential generated between the junctions directly into a indication of respiration.
Regardless of whether the thermal respiration monitoring system employs a thermistor or thermocouple as the temperature-sensitive element, conventional system typically employ only one such sensor. For example, a single respiration sensor is often taped to the individual's upper lip in the path of inhaled and exhaled air. Alternatively, arrangements have been developed in which a single sensor is supported in the respiration airflow path by a device that is partially received within the individual's mouth.
Unfortunately, conventional thermal respiration-monitoring systems have a number of shortcomings. For example, many such systems employ sensor assemblies that are relatively complex, including detailed structures for channeling airflow and supporting the sensor element. The complexity of such sensor assemblies also results in relatively high sensor cost.
Further, because conventional systems typically employ a single sensor, they are often only partially effective at sensing respiration. In that regard, respiration may pass through the individual's mouth or one or the other of the individual's two nostrils, or some combination of these passageways. As a result, a single sensor positioned, for example, in front of the individual's mouth may be unable to detect respiration from the nostrils.
In addition sensors used to monitor the respiration of a sleeping individual are often bulky or must be taped to the individual's face. As will be appreciated, in addition to being uncomfortable, sensors constructed in this manner and for use in sleep therapy may easily interfere with the individual's sleep and, hence, the respiration pattern to be monitored.
In view of these observations, it would be desirable to provide a sensor, for use with a thermal respiration-monitoring systems, that is simple, inexpensive, and responsive to various alternative airflow paterns and that does not interfere with the sleep of an individual whose respiration is being monitored.